Conventional imaging radars operate with a single fixed-polarization antenna for the radio frequency signal transmission and reception. As a consequence, for every resolution element (pixel) in the image, the scattered wave (a vector quantity) is measured as a scalar quantity and any additional information about the surface contained in the polarization properties of the reflected signal is lost. To insure that all the information in the reflected signal is retained, the reflected signal must be measured as a vector, i.e., both the amplitude and the phase should be measured. The greater information derived from the polarized measurements provides a more complete description of the backscatter phenomena of the target area. This greater information can, for example, be used in target discrimination, target classification or feature enhancement. This invention relates to techniques for utilizing polarization information to enhance certain characteristics in SAR images.
Determination of the optimal polarization state to enhance an image has been extensively studied in the past. The scattering matrix co-polarization and cross-polarization nulls represent a solution when the returns from a single point target need to be minimized. A. B. Kostinski and W. M. Boerner, "On the Polarimetric Contrast Optimization," IEEE Trans. Antennas and Propagation, Vol. AP-35, No. 8, pp. 988-991, August 1987, based their analysis on the Graves power matrix to determine the optimum transmit and receive polarizations. This technique is only applicable to maximize the contrast between two specified point targets and has been applied to polarimetric radar images by estimating an equivalent scattering matrix representation for an extended area in an image. B. James, A. B. Kostinski and W. M. Boerner, "Polarimetric Matched Filter for POLSAR Image Interpretation of Ocean Surface Scatter," Proc. IGARSS '88, P. 67, Edinburgh, United Kingdom.
The accuracy of this technique for extended targets is unknown since representation requires either an average Stokes matrix [J. J. van Zyl, A. Zebker and C. Elachi, "Imaging radar polarization signatures: Theory and observation," Radio Science, 22(4), pp. 529-543, July/August 1987; and J. J. van Zyl, C. H. Papas and C. Elachi, "On the optimum polarizations of incoherently reflected waves," IEEE Trans. on Antennas and Propagation," Vol. AP-35, No. 7, July 1987] or an average covariance matrix [J. A. Kong, A. A. Swartz, H. A. Yueh, L. M. Novak and R. T. Shin, "Identification of Terrain Cover Using the Optimum Polarimeter Classifier," J. Electromagnetic Waves and Applications, Vol. 2, No. 2, pp. 171-194, 1988]. Both these representations use the second order statistics of the scattering matrix. G. A. Ioannidis and D. E. Hammers, "Optimum Antenna Polarizations for Target Discrimination in Clutter," IEEE Trans. Antennas Propagat., Vol. AP-27, No. 3, May 1979, introduced a method based on Lagrangian multipliers to solve for the optimal polarization using the Stokes matrix. However, for some cases their solutions violate the constraint that the Stokes vector for the receive antenna must be fully polarized. Additionally, A. A. Swartz, H. A. Yueh, J. A. Kong, L. M. Novak and R. T. Shin, "Optimal Polarizations for achieving Maximum Contrast in Radar Images," J. Geophys. Res., Vol. 93, No. B12, pp. 15252-15260, December 1988, developed a parallel method based on the covariance matrix. However their analysis is restricted to the backscatter case.